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William R. Graves' and Wilhelmina van de Poll

The capacity to form nitrogen-fixing symbioses with rhizobia is common among species in the Papilionoideae subfamily of the Leguminosae, but nodulation and nitrogen fixation have never been documented in Cladrastis kentukea (Dum.-Cours.) Rudd (American yellowwood). The purpose of this study was to test the hypothesis that C. kentukea is nodulated by rhizobia. Seedlings were grown in sterile vermiculite and irrigated with a nitrogen-free nutrient solution. In one experiment, the vermiculite was inoculated with rhizobia that nodulate Maackia amurensis Rupr. & Maxim., a closely related tree species. During a second experiment, the vermiculite was inoculated with samples of soil collected near trees of C. kentukea in a native stand in Alexander County, Illinois. There were no nodules on roots of seedlings harvested 6 weeks after inoculation in either experiment. These results represent strong additional evidence that C. kentukea does not form nitrogen-fixing symbioses with rhizobia.